The present invention relates to improvements in methods and apparatus for scoring or forming grooves in a sheet of thermoplastic material which must be folded at the groove such as to form the corner window of a carton.
In the manufacture of certain cartons, the cartons are made of boxboard and have a window therein to view the carton contents which is a desirable structure for merchandising. While a single transparent window panel may be included in the carton, it is far more desirable to have a double paneled window corner so that two sides of the contents may be observed through the walls of the box providing an improved three dimensional effect for the customer. In the manufacture of such boxes, the window panel is formed of a clear or transparent thermoplastic such as PVC which is cemented to the boxboard walls of the box and folded. If the transparent PVC is folded without preparation, the plastic material in the region of the fold line tends to fold very unevenly and curve detracting from the appearance and strength of the box and providing a box which is more difficult to handle. Further, in some cases in folding of the PVC, the plastic material in the region of the fold line tends to turn white upon bending. Various methods have been proposed for preparing the clear window material such as by prescoring the material which is discussed in U.S. Pat. No. 3,379,814. Another arrangement has been proposed in U.S. Pat. No. 3,292,513 which also involves scoring the material in a cold scoring process. U.S. Pat. No. 4,946,430 discloses a further method wherein the material is heated but the heat is applied radiantly with the entire surface heated in the area of the scoring lengthwise of the travel of the path of a sheet with the scoring of the sheet after the sheet has been adequately heated over an area. In the latter arrangement, difficulty will be encountered in limiting the heat to a narrow area so that unnecessary heat energy is required plus the material is affected by heating beyond the sides of the score line.
In the formation of score lines in thermoplastic sheets, it is essential that the depth and width of the score line be positively controlled. If the score line is made too deep, the operation can penetrate the plastic so that an opening results in the side of the box which is formed by folding the window material. If the depth of the score line is inadequate, the plastic window panel will not fold as it should. Also, if the score line is applied nonuniformly in depth, the transparent window panel will not fold uniformly. Also, if too much heat is applied over a width beyond the limits of the score line, the plastic at the side of the score line is adversely affected making it difficult to control the exact dimensions of the score line.
In the formation of score lines or fold lines by heat, the displacement of the material in the formation of the score lines creates a shoulder beside the score line. This shoulder detracts from the appearance of the plastic and has not been avoided by arrangements heretofore disclosed.
These and other substantial efforts toward providing score lines indicate difficulties encountered in the prior art.